1. Field of the Invention
The present invention relates to an adaptive array antenna system and to a method of controlling the directivity of the antenna system and, more particularly, to an adaptive array antenna system of simplified construction and to a method of controlling the directivity of this antenna system.
2. Description of the Related Art
With respect to an adaptive array antenna including an array of antenna elements, the directivity can be modified dynamically according to variations in electromagnetic circumstances. Such adaptive array antennas are employed in base stations for cellular phones.
The fundamental structure of an adaptive array antenna having analog phase-amplitude change modules (load) is shown in FIG. 7. In this figure, the frequencies of signals received by antenna elements AN1, AN2, . . . , ANM are lowered to given values by downconverters DC1, DC2, . . . , DCM, respectively. Furthermore, the analog signals are converted into digital signals by AD converters ADC1, ADC2, . . . , ADCM, respectively, and supplied to a phase shift-amplitude calculation module 902 of a phase shift-amplitude control module 900. For example, the phase-shift amplitude control module 900 may be made of a digital signal processor (DSP). The phase-shift amplitude calculation module 902 performs given computation while referring to a reference signal outputted from a reference signal output module 904. The results of the computation are supplied to phase-amplitude change module PAR1, PAR2, . . . , PARM via phase-amplitude determination module PAD1, PAD2, . . . , PADM, respectively. In the phase-amplitude change module PAR1, PAR2, . . . , PARM, the phases and amplitudes of the signals received by the antenna elements AN1, AN2, . . . , ANM are varied. The modified signals are added up by a combiner 910. The sum signal from the combiner 910 is converted into a signal of a given frequency by a downconverter DC, converted into a digital signal by an AD converter ADC, and supplied to a demodulator 912.
FIG. 8 shows an example of digitized version of DBF (digital beam-forming) of a phase shift-amplitude control module 950. This beam-forming module accepts signals from antenna elements directly and obtains desired directivity by digital signal processing. As shown in FIG. 8, the phase shift-amplitude control module 950 includes phase-amplitude determination module QAD1, QAD2, . . . , QADM, phase-amplitude change module QAR1, QAR2, . . . , QARM, and a combiner 952 which have been all digitized, together with the phase shift-amplitude calculation module 902 and reference signal output module 904. The sum output from the combiner 952 is supplied to a demodulator 912.
As described so far, in the adaptive array antenna, signals received from the antenna elements are entered to the phase shift-amplitude control module, in order to control the phases and amplitudes of electromagnetic waves received by the antenna elements. However, to permit the signals to be entered to the phase shift-amplitude control module from the antenna elements, the same numbers of downconverters and AD converters as the antenna elements are required. This increases the circuit scale.
A known technique for reducing the circuit scale by omitting such downconverters and AD converters is an “adaptive array antenna” described in JP-A-2001-160708 described below. This controls the amount of phase shift based on a partial differential coefficient relative to the amount of phase shift of an evaluation function by resetting the amount of phase shift of one of plural phase shifting modules to a value obtained by increasing the presently set amount of phase shift by a given angle of 90°, then resetting the presently set amount of phase shift to a value obtained by reducing the presently set amount of phase shift by the given angle, detecting variations in intensity of the combined reception signal by a signal intensity detection module, and finding the partial differential coefficient relative to the amount of phase shift of the evaluation function using only the variations in intensity of the detected reception signal.
According to the technique of JP-A-2001-160708, the found partial differential value of phase is optimized, for example, based on a method of steepest descent. This eliminates downconverters and AD converters which would have been heretofore mounted on individual antenna elements. Furthermore, interconnect lines for them are not present. Consequently, the circuit can be simplified and made smaller in size.
However, in the known technique as described above, input signals from the individual antenna elements are not directly obtained. Rather, phase control is provided by finding a partial differential coefficient regarding the amount of phase shift without using the input signals. Therefore, the presently adopted algorithm that is used to perform calculations using the input signals cannot be used intact. In particular, it is difficult to utilize a method other than the gradient method as a method of optimizing antenna characteristics. That is, there is the inconvenience that it is impossible to perform complex manipulations such as selecting an optimization method according to the electromagnetic circumstances or it is difficult to adopt a novel algorithm for the array antenna.